The present invention relates to a method of making cemented carbide. By combining microwave sintering and coating of the WC with binder phase and no milling a cemented carbide with extremely even structure is obtained.
Cemented carbide is generally produced by powder metallurgical methods including milling of a powder mixture forming the hard constituents and the binder phase, pressing and sintering. The milling operation is an intensive milling in mills of different sizes and with the aid of milling bodies. The milling time is of the order of several hours up to several days. Such processing is believed to be necessary in order to obtain a uniform distribution of the binder phase in the milled mixture.
There exist alternative technologies to intensive milling for production of cemented carbide, for example, use of particles coated with binder phase metal. The coating methods include fluidized bed methods, solgel techniques, electrolytic coating, PVD coating or other methods such as disclosed in e.g. GB 346,473, U.S. Pat. No. 5,529,804 or U.S. Pat. No. 5,505,902. Coated carbide particles can be mixed with additional amounts of cobalt and other suitable carbide powders to obtain the desired final material composition, pressed and sintered to a dense structure. The sintering is generally made in electrical furnaces of continuous or batch type. Other methods also exist. One such method is microwave sintering known for some time, e.g., through DE 196 01 234, WO 96/33830 and WO 98/04373.
It has now surprisingly been found that cemented carbide bodies sintered in a microwave field made from powder mixtures with cobalt coated hard constituents with narrow grain size distributions and without conventional milling have a different structural profile including more narrow grain size distributions and less pronounced binder phase pools compared to corresponding powder mixtures sintered according to standard practice. Furthermore, it has been found that due to the very uniformly distributed binder phase on the carbide particles, it is possible to use microwave sintering with shorter sintering times and lower temperatures for the coated powders compared to conventionally milled powders and still get a dense structure.
According to one aspect, the present invention provides a method of making a cemented carbide comprising: providing a powder forming hard constituents; coating the hard constituent powder with binder phase material; deagglomerating the coated powder; wet mixing the coated powder with additional constituents such that no change in grain size or grain size distribution of the hard constituent powders is produced; drying the mixture; forming a green body with the dried mixture; and sintering the body in a microwave field at a temperature of 1325-1410xc2x0 C. for approximately 5-15 minutes.